You might want to try this: Go to the channels tab and alt-click (or control-click, I am not sure on a pc) on the rgb or greyscale channel, to make a selection of the black and white in your image. In the case of a line drawing you will select the lines, but you will also respect the anti-aliasing, by 'half selecting' areas that are not completely black, nor completely white. When you invertyour selection you have effectively a selection of the black lines or and areas. You can thicken it. When you use the original selection you can thin it out. In the menu you will find 'select' and further below the option to, well, grow or shrink that selection. When you are drawing on a transparent layer, you can do the control-click-thingy on that layer, without searching for that channel tab. Good luck!